This invention relates to the field of semiconductor light emitting devices, and in particular to bulbs that provide the appearance of candles.
Natural candle light is attractive. Electric candles are often preferred for a number of reasons, not the least of which is the risk of fire that is associated with an open flame.
Conventional incandescent bulbs are often designed to emulate candles because, like candles, they produce light by thermal emissions. However, incandescent bulbs are very inefficient with respect to energy utilization.
Semiconductor light emitting devices are highly efficient, but the design constraints associated with such devices, such as the need for a heat sink, limit their ability to emulate natural candle light.
When the wick of a natural candle is ignited, the light emanates from a ‘point source’ at the wick, suspended above the candle. Characteristically, less light is emitted above the wick than radially, around the wick. Conventional incandescent bulbs are configured to suspend the filament above the base of the bulb, thereby emulating the point source emission of a natural candle, and the bulb is shaped to reduce the amount of light emitted from the top of the bulb.
Semiconductor light emitting devices that may be used to emulate candle light, on the other hand, require heat sinks to dissipate the heat produced in order to extend the life of the light emitting device (LED). This heat sink requirement hinders the use of such devices as emulators of candle light. WO2010/079436, “LIGHT SOURCE WITH LEDS, LIGHT GUIDE AND REFLECTOR”, published 15 Jul. 2010 for Boonekamp et al. discloses the use of a light guide and reflector to elevate the apparent light source above the base, and is incorporated by reference herein.
As illustrated in FIG. 1, the LED(s) 110 is mounted on a heat sink 115 in the base of the bulb, and a light guide 120 is used to direct the light from the LED(s) 110 to a reflector 130 that is situated above the base. The reflector 130 is generally conical, with the apex of the cone pointed toward the LED. In this manner, the light from the reflector appears to be produced by a light source above the base of the bulb in a predominantly radial direction relative to a central axis 150 of the bulb. The profile of the reflector 130 may be concave or convex, depending upon the desired light distribution. Additionally, the reflector 130 may include an interference or diffusion coating, and may allow for some transmission through the reflector.
Although the use of a conic reflector provides the appearance of a radial point source of light above the base of the bulb, it does not truly emulate the appearance of a natural candle light.
Other techniques are known for emulating candle light. USPA 2010/0097821, “LIGHT EMITTING DIODE-BASED LAMP HAVING A VOLUME SCATTERING ELEMENT”, published 22 Apr. 2010 for Huang et al., for example, discloses a light guide that guides the LED light to a spherical scattering element that includes particles suspended in a transparent base material. By controlling the size, refractive index, and density of the particles, different light patterns can be achieved; a larger density of particles will produce more reflections, and the majority of light will be reflected downward.
Although the use of a scattering device provides for a more ‘natural’ light output, it does not provide the predominantly radial emanation desired for emulating candle light.